blacklist path + fix blacklist extension

[ksfi]

This PR addresses issues #87 and #124

It allows to blacklist paths by adding a —blacklist_path flag, the option accepts URLs and subpaths. It also fixes --blacklist which couldn’t take something without a period (like “—blacklist .png“ was working but not “—blacklist png“). I created a Blacklisted function in output.go to handle both blacklist and blacklist_flag.